Fuel cell

ABSTRACT

A simple, inexpensive and highly efficient fuel cell has boundary structures made of a photo-sensitive material in combination with selective patterning. Printed circuit board (PCB) fabrication techniques combine boundary structures with two and three dimensional electrical flow path. Photo-sensitive material and PCB fabrication techniques are alternately or combined utilized for making micro-channel structures or micro stitch structures for substantially reducing dead zones of the diffusion layer while keeping fluid flow resistance to a minimum. The fuel cell assembly is free of mechanical clamping elements. Adhesives that may be conductively contaminated and/or fiber-reinforced provide mechanical and eventual electrical connections, and sealing within the assembly. Mechanically supporting backing layers are pre-fabricated with a natural bend defined in combination with the backing layers&#39; elasticity to eliminate massive support plates and assist the adhesive bonding. Proton insulation between adjacent and electrically linked in-plane cell elements is provided by structural insulation within the central membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/435,610, filed May 9, 2003 and now issued as U.S. Pat. No. 7,341,800,which is incorporated herein by reference. U.S. patent application Ser.No. 10/435,610, filed May 9, 2003 claims the benefit of U.S. PatentApplication No. 60/379,524 filed May 9, 2002, U.S. Patent ApplicationNo. 60/408,732 filed Sep. 06, 2002, and U.S. Patent Application No.60/458,116 filed Mar. 26, 2003, all of which are hereby incorporated byreference.

FIELD OF INVENTION

The invention relates generally to fuel cells, and more specifically tofuel cells preferably constructed from photo-patterned laminates,preferably bonded with fiber-reinforced adhesive and having massivelyparallel distribution channels.

BACKGROUND OF INVENTION

A fuel cell is an electromechanical device that produces electricalcurrent from chemical reactions. The essential form of a fuel cellincludes an ion-conducting electrolyte between two electrodes that arebacked by fuel and oxidant flow distributors. A catalyst on oneelectrode, i.e. the anode, promotes separation of ions and electrons atthe fuel side. It is only the ions that pass through the electrolytewhich then at the oxidant site, i.e. the cathode, recombine withelectrons. The electrons are conducted through an external circuittherewith supplying electrical power. Excellent overviews of fuel celltechnology can be obtained from the following references:

-   -   “Fuel Cell Systems Explained” edited by J. Larminie and A. Dicks        and published by John Wiley and Sons (2000) or in a book        entitled “Fuel Cell Technology Handbook” edited by G. Hooger and        published by CRC Press (2003);    -   Related exemplary teachings can be found in U.S. Pat. No.        5,641,586, issued to Wilson, Jun. 24, 1997, titled “FUEL CELL        WITH INTERDIGITATED POROUS FLOW-FIELD”;    -   U.S. Pat. No. 5,683,828, issued to Spear et al., Nov. 4, 1997,        titled “METAL PLATELET FUEL CELLS PRODUCTION AND OPERATION        METHODS”;    -   S. J. Lee, S. W. Cha, Y. C. Liu, R. O'Hayre, F. B. Prinz, “High        Power-Density Polymer-Electrolyte Fuel Cells by        Microfabrication”, in Micro Power Sources, K. Zaghib and S.        Surampudi (eds.), Proceedings, V. 2000-3, The Electrochemical        Society Proceeding Series, Pennington, N.J., 2000;    -   S. J. Lee, S. W. Cha, R. O'Hayre, A. Chang-Chien, F. B. Prinz,        “Miniature Fuel Cells with Non-Planar Interface by        Microfabrication”, in Power Sources for the New Millennium, M.        Jain, M. A. Ryan, S. Surampudi, R. A. Marsh, and G. Nagarajan        (eds.), Proceedings, V. 2000-22, The Electrochemical Society        Proceeding Series, Pennington, N.J., 2000;    -   R. O'Hayre, T. Fabian, S. J. Lee, F. B. Prinz, “Lateral Ionic        Conduction in Planar Array Fuel Cells”, Journal of the        Electrochemical Society, Volume 150, Number 4, April 2003, pp.        A430-A438; and    -   S. J. Lee, A. Chang-Chien, S. W. Cha, R. O'Hayre, Y. I. Park, Y.        Saito, F. B. Prinz, “Design and Fabrication of a Micro Fuel Cell        Array with ‘Flip-Flop’ Interconnection”, Journal of Power        Sources, Volume 112, Issue 2, November 2002, pp. 410-418.

Prior art FIG. 1 shows a cross-section side view of a conventional fuelcell assembly. The assembly includes a membrane electrolyte 9 withcatalyst-loaded gas diffusion layer 2 on either major face of membraneelectrolyte 9. The primary electrochemical reaction occurs at theinterface between membrane electrolyte 9 and its adjacentcatalyst-loaded gas diffusion layer 2. The membrane-electrode assemblyis interposed between two backing layers 30. Backing layers 3 aremanufactured to allow for open passages 31 for reactant flow. The openpassages in conventional fuel cells are either parallel or seriallyarranged distribution channels that distribute the reactant fluid alongthe gas diffusion layer 2. For optimum operation it is desirable todistribute the reactant fluid as evenly as possible with minimalpressure and flow variances while keeping the design space for thedistribution channels to a minimum. The present invention addresses thisneed.

In order to keep the different components together and isolate reactantsat either side of membrane electrolyte 9, prior art fuel cells employ amechanical clamping structure that may include threaded fasteners 4 suchas bolts and/or nuts and other well-known mechanical clamping elementssuch as plates 41 and the like. In prior art fuel cells, the mechanicalcompression of the fuel cell's core structure within the innerboundaries of a surrounding elastomer gasket 5 may influence the fuelcell's efficiency. It is noted that the elastomer gasket 5 is positionedbetween the membrane electrolyte 9 and backing layers to isolatereactants on either side of membrane electrolyte 9. Consequently, themechanical structure involved for providing the required compressiveforce and for evenly distributing that compressive force across the fuelcell's core structure increases the fuel cell's over all size,complexity and fabrication cost.

An alternative approach has been introduced in the prior art to keep thedifferent components together and to isolate reactants at either side ofmembrane electrolyte 1 without relying on a clamping mechanism. In thisalternate approach pure adhesives are used. However, bonding of amembrane electrolyte with pure adhesives introduces severe assemblychallenges due to compatibility issues between the membrane electrolyteand the pure adhesive. Furthermore, pure adhesives are known to failreadily when subject to expansion of the membrane electrolyte as well asvarying degrees of moisture content. Accordingly, there is a need in theart for a fuel cell assembly that may be efficiently fabricated andoperated without need of a mechanical clamping structure and that takesinto account the limitations of pure adhesives used for bonding fuelcell elements. The present invention addresses this needs.

SUMMARY OF THE INVENTION

Several aspects of the invention contribute to an improved fuel cellthat simple and inexpensive to fabricate and highly efficient inoperation. The main aspects are:

-   -   use of a photo-sensitive material in combination with selective        patterning for building two and three dimensional boundary        structures for fluid conductance;    -   use of printed circuit board (PCB) fabrication techniques to        combine boundary structures with two and three dimensional        electrical flow path;    -   alternating and/or combined application of photo-sensitive        material and PCB fabrication techniques for making micro-channel        structures and micro stitch structures for substantially        reducing dead zones of the diffusion layer;    -   use of adhesive bonding for a fuel cell assembly without        mechanical clamping elements, for electrically conductive        connections by adding a metallic compound to the adhesive;    -   fiber-reinforcing the adhesive for increased stiffness of        bonding areas, building structurally supporting seals and for        fiber penetration of the central membrane;    -   pre-fabricating mechanically supporting backing layers with a        natural bend defined in combination with the backing layers'        elasticity to eliminate massive support plates and assist the        adhesive bonding;    -   providing proton insulation between adjacent and electrically        linked in plane cell elements by interrupting potential proton        flow along the central membrane. Interruption is provided by        insulation structures of adhesive, laminate and/or        photo-sensitive material.

A fuel cell is provided comprising two electrodes and an electrolytelayer interposed between the two electrodes. The fuel cell alsocomprises one or more backing layers in contact with the electrodes. Atleast one of the fuel cell has a boundary structure made of radiationcuring resin that is geometrically defined by a radiation-sensitiveimage transfer method such as, for example, selective patterning. Thecuring irradiation may be UV-light or a proton beam.

In a first aspect of the invention a backing layer of fuel cell isprovided with fluid conductance system that may include up to severallayers of independently shaped in plane and/or cross plane vacantpassages, eventual integral flow restrictions, and/or eventual valvemechanisms at least partially formed by boundary structures. Theboundary structures are preferably made by depositing and shaping ofirradiation curing resin in eventual combination with etching,electroplating, sputtering, electrodeposition, printed circuit board(PCB) fabrication techniques or any other well known fabricationtechnique for micro scale and macro scale structural elements.

The one or more backing layers preferably contain prescribed highelectrical-conductivity regions and prescribed lowelectrical-conductivity regions. In one embodiment, the high and lowelectrical-conductivity regions are preferably defined by a selectivedeposition process wherein a broadly conductive layer is selectivelydeposited through unobstructed regions of a mask. Alternatively, thehigh and low electrical-conductivity regions are preferably defined by aselective etch process wherein the broadly conductive layer isselectively etched through exposed regions of a mask in contact with theconductive layer.

Across the assembly direction of the fuel cell's layers, ports orthrough holes may be fabricated as well. Such ports may be utilized forfluid conductance between individual functional layers. Thehigh-electrical-conductivity regions may extend along the side walls ofsuch ports in a fashion well-known for PCB vias. The through hole viaconnections may also be positioned in close proximity to, or directcontact with, the electrodes. The high electrical conductivity regionsare preferably in contact with other components of the fuel cell withoutrequiring external mechanical compression. The contact is preferablyelectrical contact. The high electrical conductivity regions preferablyform a continuous electrically conductive path through the bulk of amaterial having substantially lower electrical conductivity, therebyproviding a prescribed path of electrical conductance. The centralmembrane may also be selectively deactivated and/or structurally alteredin regions between adjacent cell elements by prescribed mechanical,thermal, chemical, or electrical degradations and/or alterations, suchthat ion conductivity between individual cell elements is substantiallyinhibited within the fuel cell assembly.

A number of individual fuel cell elements may be in plane assembled andelectrically connected within the fuel cell assembly. The electricalconnections may be parallel and/or serial and reconfigurable by suitableinsertion or removal of electrically conductive junction elements suchas, for example, jumpers, switches, and solder joints.

Preferably, the fuel cell's layers are held together without substantialcompressive force externally applied. No mechanical clamping elementsare part of the fuel cell assembly. Individual layers are held togetherby adhesive bonding. During the bonding process, the bonded layers aretemporarily compressed across the areas of applied adhesive. Once theadhesive has cured, the temporary compressive force is released andsubstituted to a certain extend by a resilient tensile force between theadhesively bonded areas. To evenly distribute the tensile force acrossareas without direct adhesive bonding, the backing layers may beprefabricated with a predefined curvature that corresponds somewhat witha dome. The dome may be shaped in conjunction with the backing layersnatural resilience such that it provides a substantially evencompression onto the adjacent planar layer once bonded to that adjacentlayer.

Adhesive substance may be used for bonding purposes and/or for locallyincreasing conductivity between layers, for example, to make currentcollectors or electrical interconnections between cell elements. Theadhesive may be composed of an inorganic material, and may also form ahermetic seal around prescribed cavities.

A method of producing a fuel cell is includes preparing a semi-rigidpolyimide substrate with patterned through-holes for reactant gas portsand electrical interconnections. A photo-sensitive epoxy resin (e.g.,MicroChem SU-8) is coated on the polyimide substrate, and issubsequently patterned with reactant flow channels by photolithographythrough a predefined mask. The surface of the epoxy structures isoptionally coated by a conductive film. A photo-chemically etched metalfoil is then laminated onto the flow channels, where the etched openingsin the metal foils are small, preferably on the order of 10 microns. Thefine feature size serves dual purposes as a structural support to holdthe catalyst material in place, and as a fine electrically conductivegrid to conduct current with low resistance. A membrane-electrodeassembly having a carbon-supported platinum catalyst layer dispose oneach side is then bonded between symmetric constructions of the metalfoil plus photo-patterned channels plus polyimide backing.

In a second aspect of the invention a fuel cell assembly including afiber-reinforced adhesive to bond fuel cell components is introduced. Inaddition to facilitate bonding of fuel cell components, thefiber-reinforced adhesive also serves to seal and isolate reactants inthe chemical reaction means of the fuel cell. The fiber-reinforcedadhesive is typically compliant in nature, compatible with the membraneelectrolyte and includes an adhesive as well as a network of fibers. Amembrane electrolyte is bonded to backing layers using thefiber-reinforced adhesive. The backing layers are bonded by the adhesiveproperties of the fiber-reinforced adhesive. The electrolyte is bondedby the adhesive properties of the fiber-reinforced adhesive as well asthis bond is strengthened by penetration of the fibers of thefiber-reinforced adhesive into the electrolyte.

The assembly of fuel cell components using a fiber-reinforced adhesiveoccurs under elevated temperature and under elevated pressure. Thefiber-reinforced adhesive could come in a one or more patterned sheet orin a free-flow form. The bonding of two or more fuel cell componentscould occur simultaneously and in an automated fashion.

The ultimate fuel cell assembly of the present invention does notrequire external compression during operation since the mechanical bondand strength to keep together the fuel cell assembly during operation isnow provided by the bonding characteristics and strength of thefiber-reinforced adhesive.

The introduction of fiber reinforcement in the fiber-reinforced adhesiveprovides a more reliable bonding compared to pure adhesives.Particularly, the use of fiber-reinforced adhesive additionally supportsa compact and lightweight packaging that is enabled by the use ofinherently thin manufacturing materials, and especially by eliminatingthe need for conventional load-bearing components such as threaded boltsand rigid compression plates. Automated fabrication to assemble the fuelcell of the present invention is favored by the use of highly scalablemanufacturing processes including continuous compression plates. Designflexibility and complexity of the fuel cell assembly are enabled by thefact that the fiber-reinforced adhesive is patternable into intrinsicpatterns. Also, increased durability of the fuel cell assembly isprovided by the compliant nature of fiber-reinforced adhesive. Theassembly in effect becomes an engineered laminate composite and canthereby be optimized in terms of stiffness, toughness and othermechanical properties. Lower cost is provided by the choice ofbatch-process materials and non-manual assembly sequences.

In a third aspect of the invention a new design concept is described forreactant flow distribution in a fuel cell, in such a way as to promoteuniform pressure and velocity, minimize fluid dynamic losses, and boosthigh cell performance. Small flow paths are arranged in a prescribedmanner to deliberately control flow characteristics at each electrodesite. The local channels control the degree of active (forced volumedisplacement) and passive (diffusion-driven) flow. Larger channelsinterface with the small channels, such that pressure loss is minimizedand velocity uniformity is maintained. Following new features areprovided: enabling of exceptionally small, massively parallelmicro-channels for fuel cells, without suffering high pressure loss;broad-area uniformity by using interdigitated or otherwise alternatingchambers for supply and exhaust; and methods of scalable, automated,low-cost manufacturing. The new features provide the followingadvantages over the prior art:

-   -   Uniform pressure and flow are achieved because the gross        distribution of reactant is made in large channels to completely        span the active cell area, and smaller channels having high        resistance exist only very locally with short travel length.    -   Reduced pressure loss is accomplished because the smaller        precise channels are highly parallel and represent only a small        fraction of the overall travel length from supply to exhaust.    -   Performance optimization can be controlled to an exceptionally        fine degree because the flow behavior is locally controlled on a        sub-millimeter scale, avoiding broad-area non-uniformity.    -   Compact and lightweight packaging is enabled by the use of        inherently thin manufacturing materials.    -   Automated fabrication is favored by the use of highly scalable        manufacturing processes including photo-patterning, and        continuous film or batch sheet processing.    -   Lower cost is provided by the choice of batch-process materials        and non-manual assembly sequences.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a simplified cross section of a prior art fuel cell.

FIG. 2 depicts a simplified cross section of a fuel cell according to anobjective of the present invention.

FIG. 3 shows a first exemplary cross section of a simplified backinglayer configured with micro-channels.

FIG. 4 is a bottom to top view of an exemplary backing layer similar tothat depicted in FIG. 3.

FIG. 5AA illustrates a first simplified section view indicated in FIGS.3, 4 by section line A-A.

FIG. 6AA illustrates a second simplified section view indicated in FIGS.3, 4 by section line A-A.

FIG. 7 shows a second exemplary cross section of a simplified backinglayer configured with micro-channels.

FIG. 8 shows a third exemplary cross, section of a simplified backinglayer configured with micro-stitch.

FIG. 9 shows a fourth exemplary cross section of a simplified backinglayer configured with micro-stitch.

FIG. 10 shows a central cross section of a simplified fuel cell forillustrating the fluid flow for a micro-stitch backing layer.

FIG. 11 shows a bottom to top view of a simplified backing layer forillustrating the fluid flow for the micro-stitch backing layer.

FIG. 12 depicts a perspective exploded view of an exemplary fuel cellassembly.

FIG. 13 illustrates a simplified section view of a fuel cell in preassembly condition with the backing layers shown with a natural bend.

FIG. 14 shows a schematic cross section of a fiber-reinforced adhesivelayer.

FIG. 15 shows a schematic cross section of two layers bonded with thefiber-reinforced layer.

FIG. 16 illustrates the effect of proton cross conductivity by showing aschematic cross section of a membrane-electrolyte assembly with twoadjacent and electrically linked in-plane cell elements.

FIG. 17 is a schematic cross section of a membrane-electrolyte assemblycorresponding to FIG. 16 with exemplary proton insulation structures.

FIG. 18 shows an exemplary graph of cell voltage versus current densityfor various micro channel sizes.

FIG. 19 shows an exemplary graph of cell voltage versus feature size forvarious levels of current density, illustrating that the voltage ishigher for smaller feature size at any given current density.

FIG. 20 shows an exemplary graph of power density versus current densityfor two micro channel sizes.

FIG. 21 shows an exemplary graph of peak power density and power lossversus feature size, and highlights on the upper curve that peak poweris higher for smaller feature size, in contrast with a relativelysmaller penalty from power loss associated with driving fluid throughvery narrow channels.

FIG. 22 shows an exemplary graph of current drain resistance versusinter layer pressure for the cases with and without Ag enriched resinbonding of the diffusion layer to the backing layer.

FIG. 23 shows an exemplary graph of cell element voltage versus currentdensity for the cases of traditional compressed and uncompressedconductively bonded layer assembly.

FIG. 24 shows an exemplary graph of power density versus current densityfor the cases of traditional compressed and uncompressed conductivelybonded layer assembly.

FIG. 25 shows an exemplary graph of operational voltage for a first cellversus the voltage potential of an adjacent second cell. The graph linesillustrate the relation between the two adjacent cells for varyingproton insulation conditions between the two of them.

FIG. 26 shows an exemplary graph of adjacent cell distance over cellborder area for varying maximum power reductions and maximum operationalcell voltages.

DETAILED DESCRIPTION

In the present invention novel design configurations and fabricationmethods for compact fuel cells with high power density are described.Referring to FIG. 2, a basic fuel cell device 1 in accordance with thepresent invention includes an ion-conducting electrolyte membrane 10between two electrodes 20, backed by backing layers or backingstructures 30. The backing structures feature flow distribution channels31, 32, 35, 36, 37, 38 (see FIGS. 3-11) for fluid supply and fluidexhaust. A catalyst on one electrode promotes separation of ions andelectrons at the fuel side. Only the ions conduct through theelectrolyte, and recombine with electrons at the oxidant side. Theelectrons are conducted through an external circuit, thus supplyingelectrical power. The layers of the fuel cell 1 are assembled along theassembly axis 101.

For ease of the invention the numerous aspects of the invention areinitially described in separate chapters. Cross dependency of theindividual aspects after the separate description of the numerousaspects.

Boundary Structures Made of Photo-Sensitive Material

In a first embodiment, a boundary structure for defining a vacantpassage for fluid conductance is fabricated from a photo-sensitivematerial, such as a UV-curing epoxy resin. The fabrication of theboundary structure is accomplished by use of a substrate on which thephoto-sensitive material is deposited in an uncured fashion.Photo-sensitive materials are commonly used for fabrication ofsacrificial patterns used for example for etching structures. Dependenton the stiffness and strength requirements within the fuel cellassembly, the substrate may be, for example a semi-rigid polyimide or afiber-reinforced epoxy. The substrate may be permanent or sacrificial.

Typical characteristics of a photo-sensitive material relevant forfabrication of boundary structures in a fuel cell include smallestpossible feature size, feature aspect ratio, and a number of physicalproperties. Physical properties include gas impermeability, curingshrinkage, thermal expansion, thermal conductivity, thermal resistance,elasticity, chemical fuel cell fluid resistance, and adhesive strength.A preferred material complying to the relevant characteristics is anepoxy resin commercially available under the trade name MicroChem SU-8.The making of a boundary structure is performed in several steps thatare similar to that of making a sacrificial pattern. Initially, theuncured photo-sensitive material is deposited on a substrate. It may bebacked to reduce the uncured material' viscosity such that the depositedmaterial remains substantially immovable during consecutive fabricationsteps. In that fashion the deposition process may be repeated toincrease the total deposition height beyond the level of a singledeposition process. The photo-sensitive material may be also depositedin form of a well-known prepreg.

Depending on the photo-sensitive material's curing characteristic, amaximum contour height of a single contour level may be fabricatedduring a following selective patterning combined with a irradiationcuring. The curing characteristic of MicroChem SU-8 for example providesfor a conventional UV-irradiation a minimum feature resolution on theorder of 1 micron and a minimum feature size for structural elements onthe order of 10 micron for a curing height between 10 microns and 1millimeter. Greater than 10:1 aspect ratio may be achieved, meaning thatif a feature size of 10 microns is desired, a contour level fabricatedin a single irradiation curing may have a maximum height of greater than100 microns.

Contour height of contour level fabricated without consecutive materialremoving operations is mainly defined by the deposition process duringwhich the uncured photo-sensitive material is deposited with a certainheight. Manufacturers of photo-sensitive materials typically providedetailed information about procedures for uncured material depositionswith predefined deposition heights.

In context of the present invention a contour level is that level of aboundary structure at which side walls of the boundary structure aresubstantially continuous and propagating somewhat in direction ofdeposition height. Dependent upon the irradiation source's orientationand focus, sidewalls may converge, diverge or propagate perpendicular tothe deposition direction of the boundary structure.

The use of a photo-sensitive material in combination with selectivepatterning and irradiation curing provides also for feasible massproduction of non planar boundary structures. In such cases thephoto-sensitive material may be deposited either on a non planarsubstrate or a planar substrate. A three dimensional curvature may befabricated either by depositing it on a curved substrate and/or bybaking the deposited photo-sensitive material in a curvature mold. Also,secondary machining operations may be applied to the baked but uncuredresin to fabricate a three dimensional curvature. This advantage willbecome more apparent in the below chapter of “Natural Bent BackingLayers”

At the time this invention was made, UV-irradiation for curingphoto-sensitive materials is known to the inventors as substantiallydepth insensitive. This means for the fabrication of a contour height ofa contour level, that after each irradiation step the uncured materialneeds to be removed to prevent inadvertent curing during the irradiationcuring of a consecutively deposited layer.

To the knowledge of the inventors, UV-curable resin such as MicroChemSU-8 may also be cured by proton irradiation with the particularadvantage of adjusting a curing depth by modulating proton irradiationparameters. In that way deposition, and selective patterning may beperformed in a repetitive fashion without need of intermediate removalof uncured photo-sensitive material. The uncured material remainingafter the selective patterning and irradiation curing may be utilized asa sacrificial substrate in a consecutive fabrication cycle ofdepositing, selectively patterning and irradiation curing. This serialpatterning is highly advantageous in fabricating intricately shapedlayers with a number of overlapping and/or covering contour levelsfabricated on top of each other. All uncured residual material remainingafter the serial patterning is removed during a final simultaneousdevelopment of the number of previously proton-irradiated levels. Incontext with the present invention, a shaped layer is a layer of a fuelcell assembly made from photo-sensitive material with selectivepatterning and irradiation curing.

For the reasons stated above, the use of a photo-sensitive material ishighly attractive for making boundary structures in a fuel cell. Othertechniques such as sputtering and/or electroplating may be utilized forfabricating conductive leads and/or conductive regions in combinationwith photo-sensitive materials.

The photo-sensitive material may be used for fabricating bottom,side-wall and/or top portion of a vacant passage and other structuralelements such as valves, flow regulators or electro-mechanical elements.Photo-sensitive materials with varying properties may be combined forspecific purposes as is well appreciated by anyone skilled in the art.Other structures, like for example a proton insulation structure may befabricated from photo-sensitive material as described above. The use ofa proton insulation structure is described under the below chapter“Proton Cross Conductivity Avoidance”.

Yet in some instances it is necessary to include other fabricationtechnique to include and/or integrate functional elements such asconductive leads, paths, and areas or to provide other physicalproperties not obtainable with photo-sensitive materials. Particularly,fiber-reinforcement is impractical for irradiation curing of featuresthat have similar dimensional scale as the fiber width, since the fibersintroduce a disturbance to the irradiation propagating through thedeposited material during the curing step.

PCB Fabrication Techniques for Conductive Paths Combined with BoundaryStructures

Another way of fabricating a shaped layer having multiple contour levelsis by laminating and bonding a number of independently fabricatedcontour levels and/or shaped layers. Moreover, any board like structuremay be laminated and integrated as long as it fits sufficiently forbonding purposes. Specifically for building a more massive boardstructure with larger scale features compared to those made withphoto-sensitive materials, lamination and other fabrication techniqueswell-known for printed circuit board (PCB) are introduced in combinationwith photo-sensitive materials. The board structure may be fabricatedfrom a number of laminated prepregs. Fiber-reinforced epoxy may be usedto provide stiff structures that may span across extended areas withonly minimal deflection.

The board structure may be utilized as a substrate for fabricatingcontour levels, shaped layers, and/or boundary structures fromphoto-sensitive material as explained in the chapter above. The boardstructure in itself may also feature boundary structures for example,for supply channel manifolds 31, 35, 37 and/or exhaust channel manifolds32, 36, 38 as is described in the below.

The board structure is preferably fabricated in a dimensional scalesimilar to that of well-known PCB. Hence, inexpensive and readilyavailable PCB fabrication techniques may be utilized to fabricateelectrical components suitable for collecting and/or transmittingcurrent that occurs at the diffusion layer during the fuel cell'soperation. Of particular interest are via holes usually employed in PCBfor soldering electronic components to the PCB and for providingconductive paths from one side of the PCB to the other side or to/withinmetallic layers within the PCB. Such via holes are usually through holesfabricated with varying hole diameters and featuring metal coatedsidewalls. In the present invention, via holes are utilized for fluidconductance together or alternating with collected current transmission.This is particularly advantageous for configurations in which a largenumber of tightly arranged via holes are employed for efficient fluidconductance with simultaneous current transmission across the boardstructure's height.

Another technique employed in the present invention is a well-known PCBfabrication technique for shaping metallic cladding layers present atone or both sides of the PCB as well as within the PCB. In that way,boundary structures of vacant passages as well as conductive leads orpath may be fabricated. In the particularly relevant case of fabricatingsidewalls of vacant passages adjacent a gas diffusion layer 20, thehighly conductive properties of the cladding layer are advantageous intransmitting the collected current away from the gas diffusion layer.PCB like board structures have a range in thickness between 50 micronsand 1 millimeter. PCB like cladding layers utilized for boundarystructures range in thickness between 10 microns and 200 microns. Metalsof cladding layers may be copper, which is relatively easy to patternand etch. Additional corrosion resistance against the fuel cell'soperational fluids, the cladding layer may be coated with metalliccorrosion resistant layer such as gold. Besides the cladding layer othertechniques such as sputtering and/or electroplating may be utilized forfabricating conductive leads and/or conductive regions.

To utilize conductive via holes and/or cladding layers for boundarystructures with high electrical conductivity and to accommodate forthickness limitations particularly of cladding layers on commerciallyavailable PCB raw material, special channel designs such as themicro-channel architecture and the micro-stitch architecture areembodied in the present invention.

Micro-Channel Architecture

According to FIGS. 3 and 4 a backing layer 30 provides vacant passagesin the configuration of massively parallel micro channels 33 arrayed ona contacting face 39 of the backing layer 30. The feature size of themicro channels 33 is preferably between 20 microns and 400 microns.Minimum channel size is influenced by the structural configuration ofthe adjacent and contacting gas diffusion layer 20. For a diffusionlayer 20 that includes carbon cloth, the carbon fibers have a thicknessof about 10 microns. Reducing the channel width below the fibersthickness bears the risk of clogging the top of the micro channels 33and inhibiting the fluid propagation between the diffusion layer 20 andthe micro channels.

The relatively small cross sections of the micro channels 33 arecompensated by a massively parallel arrangement of them. Fluid isconducted into and out of the micro channels 33 through first openings341 and second openings 342. The first openings 341 are vacant passagesconnecting finger channels 31 of a supply channel manifold with themicro channels 33. The supply channel manifold includes a manifold inlet37 and a supply cross channel 35. Fluid entering the fuel cell 1 throughthe manifold inlet 37 propagates along the supply cross channel 35 andis gradually distributed into the supply finger channels 31. The fluidpropagating along the supply finger channels 31 is gradually distributedvia the inlet openings 341 into the micro channels 33. Manifold inlet37, supply cross channel 35 and supply finger channels 31 are part ofthe supply channel manifold.

Residual fluid exits the micro channels 33 through outlet openings 342into exhaust finger channels 32 which direct the residual fluid into anexhaust cross channel 36. All residual fluid collected from theindividual exhaust finger channels 32 propagates towards the manifoldoutlet 38 where it exists the fuel cell 1. Manifold outlet 38, exhaustcross channel 36 and exhaust finger channels 32 are part of the exhaustchannel manifold.

The combination of interdigitated supply finger channels 31 and exhaustfinger channels 32 in combination with the cross oriented micro channels33 provides for a highly effective utilization of the fuel cell's 1 footprint for fuel cell elements. The area extension of a micro channelfield 334 is preferably defined in conjunction with the area requirementof single cell element for a given power output of that cell element. Incontext with the present invention, a micro channel field 334 may be ansubstantially continuous array of micro channels 33. Separation betweenmicro channel fields 334 is defined by proton insulation requirementsbetween electrically linked cell elements as is described in more detailin the below chapter “Cross Conductivity Avoidance”. Separation betweenmicro channel fields 334 is also defined for adhesive bonding areas asdescribed in more detail in the below chapter “Adhesive Bonding”.Separation between micro channel fields 334 is further defined forcurrent transmission as described in more detail in the below chapter“Current transmission”.

It is desirable to arrange finger channels 31, 32 and micro channels 33such that the openings 341, 342 may be at a maximum for a given with ofthe micro channels 33 and the finger channels 31, 32. For that purpose,the micro channels 33 may overlap the finger channels 31, 32 to theextent that each opening 341, 342 may extend over the entire width ofthe finger channels 31, 32 and still being within the boundaries of thecorresponding micro channel 33. The openings 341, 341 may be at the endof the micro channels 33, which means in context with the invention aposition with respect to the finite length of the micro channels 33 suchthat externally forced fluid exchange as well as well-known reactiondriven fluid exchange between the diffusion layer 20 and the microchannel 33 is substantially constant along the micro channel's 33length.

The micro channels 33 form together with the openings 341, 342 and thechannel manifolds an intricately shaped boundary structure, which mayinclude shaped layers as well as board structures. Shaped layers and/orboard structures may be implemented depending on the scale of the microchannels 33, the number of micro channel fields 334 and depending onother design criterions well appreciated by anyone skilled in the art.For example, where the micro channels 33 are at a scale compatible withPCB shaping techniques for cladding layers, a first board structure maybe employed across the micro channel contour height 330. Micro channels33 may be fabricated into the cladding layer. The openings 341, 342 maybe via holes in the first board structure or part of a second boardstructure bonded to the first board structure. The channel manifoldswith their manifold contour height 310 may be integrated within eitherthe first, second or a third board structure bonded to the adjacent one.Applicable feature sizes of openings 341, 342 and micro channels 33 maybe in the range of 20 microns and 400 microns.

In another example, where the micro channels 33 are at a scalecompatible with forming techniques discussed in the above forphoto-sensitive materials, a first shaped layer may be employed acrossthe micro channel contour height 330. A first contour level defining themicro channels 33 across the contour height 330 may either be fabricatedon a sacrificial substrate or on top of a board structure or a contourlevel providing the openings 341, 342. In case a board structure isemployed across the contour height 340, the openings 341, 342 may againbe fabricated as via holes. In case where the micro channels 33 are at ascale below dimensional PCB fabrication limits, a second contour levelmay be employed across the contour height 340. This example may be wellcontinued by anyone skilled in the art to demonstrate any possiblecombination of PCB fabrication and photo-sensitive material forproviding the boundary structures of all involved vacant passages incorrespondence with dimensional scale and optimum mass production.Applicable feature sizes of openings 341, 342 and micro channels 33 forbest use of photo-sensitive material may be in the range of 20 micronsand 400 microns.

In the case, where solely photo-sensitive material is employed formaking the boundary structure, a substrate may be provided across thebacking height 301. In that case the substrate may be of semi-rigidpolyimide or a fiber-reinforced epoxy.

Channel width and separation distance between individual micro channels33 is preferably similar, to keep the contact pressure between theboundary structure and the diffusion layer within practical ranges.Also, a certain contact area between the boundary structure and thediffusion layer 20 is desirable for an improved current collection fromthe diffusion layer 20. More details are found in the chapter below“Current Collection”.

For a substantially homogenously shaped micro channel field 334 in whichfeatures sizes such as channel width, channel height and channel spacingare substantially equal, a number of experimentally determined currentdensities for operational cell voltages are illustrated in FIG. 17. Thevarious curves where measured for features sizes in the range between 5and 500 microns. Surface roughness and other well-known influences whereconsidered constant for the measurements. The results of FIG. 17 arepresented in an inverted graph of FIG. 18, in which cell voltage isplotted over feature size. The curves in FIG. 18 are curves of constantcurrent density.

Under the same presumptions as described under FIG. 17, FIG. 19 showspower density over current density for 20 microns and 100 micronsfeature sizes. FIG. 20 again shows peak power density over feature sizefor varying current densities.

Current Collection

For efficient operation of the fuel cell 1, current is effectivelycollected at a collection interface between the contacting face 39 andthe diffusion layers 20. Efficient current collection is accomplished inseveral ways and in accordance with the nature of the boundarystructures adjacent the diffusion layers 20. In case of a boardstructure adjacent a diffusion layer 20, the contacting boundarystructure may be of solid metal made from a cladding layer. Conductivityat the contacting face 39 is high.

In case of photo-sensitive material being employed as a boundarystructure the naturally low electrical conductivity of the resin at thecontacting face 39 may be increased by conductively coating thecontacting face 39 as shown in FIG. 5AA. The metallic coating 391 may beprovided by electroplating and/or sputtering in combination with aselective patterning and/or consecutive etching such that the highlyconductive regions remain confined within areas occupied by cellelements. Electrical cross conductivity between adjacent cell elementsis prevented.

Electrical conductivity at the contacting face 39 may be also increasedby bonding a prefabricated metal foil onto the contacting face 39 as isillustrated in FIG. 6AA. The metal foil is prefabricated withperforations to make the metal foil gas permeable such that fluid mayconduct towards and away the diffusion layer.

In both cases of FIGS. 5AA and 6AA a conductive adhesive mayadditionally applied in the collection interface. Particularly for afuel cell 1 without clamping mechanisms it is desirable to keep thecontact resistance in the collection interface to a minimum regardlessthe contact pressure in the collection interface. FIG. 22 shows a graphof experimentally obtained contact resistance over contact pressure inthe contacting interface. Curve 2201 shows the results for a blankcontact interface without conductive adhesive. Curve 2202 shows theresults for an improved contact interface with conductive adhesive. As aresult of the reduced contact resistance current density and powerdensity also increase as shown in FIGS. 23 and 24. In FIG. 23, curve2301 is for the blank interface and curve 2302 is for the improvedinterface. In FIG. 24, curve 2401 is for the blank interface and curve2402 is for the improved interface.

For the experiment the adhesive a silver-loaded epoxy with volumeresistivity less than 0.001 Ohm-cm and lap shear greater than 1200lb./in. Flowfields were etched in stainless steel foil having thicknessnominally 50 microns and channel width approximately 100 microns. Flowchannels were arranged as parallel rows over an active cell area of 14mm×14 mm. Testing was conducted at room temperature and 1 atm pressure.The membrane-electrode assembly was a conventional Nafion 115 withplatinum catalyst loaded at 2 milligrams per centimeter squared.

Current Transmission

For efficient operation of the fuel cell 1, current is effectivelytransmitted away from the collection interface in several ways and inaccordance with the nature of the boundary structures adjacent thediffusion layers 20.

In case of a board structure adjacent a diffusion layer 20, thecontacting boundary structure may be of solid metal made from a claddinglayer. Conductivity across the contour height 330 is consequently high.Also, a conductive compound may be added to the board structure toincrease electrical conductivity within the board structure.

In case of photo-sensitive material being employed as a boundarystructure the naturally low electrical conductivity of the resin acrossthe contour height 320 may be increased by conductively coating the sidewalls of the micro channels 33 as shown in FIG. 5AA. The coating may beprovided in combination with a coating of the contacting interface asdescribed in the chapter “Current Collection”

The current is preferably separately transmitted away from each cellelement. For that purpose, conductive paths or leads are fabricatedacross and/or along the contour heights 330 and/or 340. FIGS. 7-9illustrate exemplary cases of photo-sensitive materials and/or boardstructures in sole or combined use.

In FIG. 7, a board structure 346 is employed across height 340. Theboard structure 346 has via holes operating as inlet and outlet openings341, 342. The via holes have conductive walls 343 conductively connectedwith bulk leads 345 at a level distant from the contacting face. Incontext with the present invention, the distant level is preferably theside away from contacting face 39. The bulk leads may be fabricated intocladding layers. On the same side as the bulk leads 345 are the channelmanifolds 31, 32. The cross sections of the channel manifolds 31, 32 areat a scale such that the height of the bulk leads 345 has substantiallyno adverse effect on the fluid propagation. The bulk leads 345 may betightly arranged for a low resistive conductance of current towards thefuel cell's 1 circumference. Across the contour height 330 microchannels 33 are provided by a shaped layer. The side walls and bottom ofthe micro channels 33 as well as the contacting interface are metalcoated. The metal coat is conductively connected with the via walls suchthat the a conductive path is established from the contacting interfaceacross heights 330, 340 and along the bulk leads 345.

In FIGS. 8 and 9, the backing layer 30 features a micro-stitcharchitecture further explained in the below chapter “Micro-Stitcharchitecture”. In FIG. 8, the entire boundary structures may be providedby a shaped layer fabricated with a number of contour levels on top of asubstrate present across the height 301. Conductive leads fortransmitting the collected current towards the fuel cell's 1circumference are deposited on the contacting face 39 together and inbetween the current collectors.

In FIG. 9, a board structure 347 provides the contacting face 39. Theremaining of the backing layer 30 is provided by a shaped layer. Bulkleads 345 are at side away of the contacting face 39 similar asdescribed under FIG. 7. Current collectors are deposited on thecontacting face 39 directly on board structure 347. Current collectorsare conductively connected with the bulk leads 345 via the conductivewalls 343.

At the circumference of the fuel cell 1, electrical elements 392 (seeFIG. 12) like, for example solder terminals, jumpers and the like may beemployed to provide temporary and/or permanent linking connections to,from and/or between individual cell elements of the fuel cell 1.

Micro-Stitch Architecture

According to FIGS. 10 and 11, a micro-stitch architecture provides for afluid flow through a diffusion layer 20 in regions between inlet holes341 and outlet holes 342 adjacent the diffusion layer 20. The inletholes 341 are arrayed with respect to the outlet holes 342 in analternating and interlaced fashion and protrude towards the diffusionlayer 20 through the contacting face 39. The fluid propagates in thevicinity of the inlet holes 341 through the diffusion layer 20substantially radially away from the inlet holes 341 and in the vicinityof the outlet holes 342 the fluid propagates through the diffusion layer20 substantially radially towards the outlet holes 342. As a result,dead zones of the diffusion layer 20 are substantially eliminated. Adead zone in the context of the present invention is a region of thediffusion layer where substantially no fluid reaches the electrolyte.Keeping the dead zones low contributes to a high all over currentdensity along a cell element's diffusion layer.

The inlet holes 341 and outlet holes 342 communicate with theirrespective supply finger channels 31 and exhaust finger channels 32. Apitch 349 between the inlet holes 341 and the outlet holes 342 may be aslow as 100 microns. The minimum size of the holes 341, 342 is limited bythe features of the catalyst-loaded gas diffusion layer, to preventblockage of the holes by either the carbon fibers or the catalystpowder. The two dimensional array of the inlet and outlet holes 341, 342may be in angle to the protrusion direction of the finger channels 31,32. In the case illustrated in FIG. 11, where the hole array angle isabout 45 degrees with respect to the protrusion direction of the fingerchannels 31, 32, the width of the finger channels 31, 32 is at a minimumfor a given pitch 349.

To provide the width of the finger channels 31, 32 more independentlyfrom the pitch 349, three dimensional manifold channel may beaccomplished in combination with shaped layers and/or board layers asdescribed in the below chapter “Spatial Manifold Arrangement”.

Spatial Manifold Arrangement

The possibility to shape intricate boundary structures by either ashaped layer or a board structure has been described in the abovechapters “Boundary Structures Made of Photo-Sensitive Materials” and“PCB Fabrication techniques for Conductive Paths Combined with BoundaryStructures”. These possibilities are combined are independently appliedto provide the distribution channel system that includes supply channelmanifolds and exhaust channel manifolds in two ways. Firstly and asexplained in the chapters above, supply channel manifold and exhaustchannel manifold may be in plane and eventually defined within a singlecontour level of a shaped layer.

Secondly and as is illustrated in FIGS. 8 and 9, supply channel manifoldand exhaust channel manifold are offset along the assembly axis 101.Thus, supply channel manifold and exhaust channel manifold may bedefined within separate contour levels of said shaped layer. In thatway, the width of the finger channels 31, 32 may be more generouslyselected. The length of the holes 341, 342 is accordingly adjusted toextend to the corresponding channel manifold.

The examples of FIGS. 8 and 9 exemplarily illustrate the inventiveconcept of utilizing several separated contour levels for fluid exchangetowards and away from the gas diffusion layer. As it may be wellappreciated by anyone skilled in the art, this concept may be modifiedto provide a fluid supply and/or fluid exhaust within separate contourlevels as is feasible to fabricate and operate.

Adhesive Bonding

Intricate boundary structures and other functional layers of the fuelcell may be bonded together by adhesive. Photo-sensitive material may bespecifically suitable for adhesive bonding when combined with chemicallysimilar adhesives. For example, an epoxy based photo-sensitive materialsuch as MicroChem SU-8 or a fiber-reinforced epoxy of a board structuremay be well combined with an epoxy based adhesive. The similar chemicalcomposition may provide well-known bonding advantages such as, forexample, increased bonding strength, similar thermal properties, similarchemical properties and similar physical properties, which in summarymake the bonding more reliable and easy to accomplish.

In addition, the use of an adhesive may reduce or eliminate separateparts such as, seals and insulators and clamping structures. Thus, animproved fuel cell 1 that utilizes adhesive bonding is more simple andinexpensive to fabricate, while providing a highly compact overalldesign and efficient operation.

As is illustrated in FIG. 12, backing layers 30 may be adhesively bondedthrough material separations 11, 12 of the membrane 10. Materialseparations 11 provide additional function as explained in the belowchapter “Proton Cross Conductivity Avoidance”.

Fiber Reinforced Adhesive

The adhesive may be additionally fiber-reinforced for reason describedin the following. FIG. 12 shows an exploded isometric view of the fuelcell assembly of the present invention, showing membrane electrolyte 10having catalyst-loaded gas diffusion layer 20 on either major face.Membrane electrolyte 10 could for instance be duPont Nafion 115 that isprepared with electrode backing material such as carbon cloth withplatinum catalyst interposed to form catalyst-loaded gas diffusion layer20. However, as a person of average skill in the art would readilyappreciate, the present invention is not limited to these types ofmaterials since other electrolyte materials, electrode materials andcatalysts could be also used without departing from the scope of theinvention. In order to provide a region to seal and isolate the reactantchambers behind each catalyst-loaded gas diffusion layer 20, membraneelectrolyte 10 itself extends to a region wider than the catalyst-loadedgas diffusion layer 20 common to the art of fuel cell design.

The exemplary embodiment of FIG. 12 further shows a patternedfiber-reinforced laminate adhesive sheet 60 on either side of membraneelectrolyte 10, which bonds membrane electrolyte 10 to each of thebacking layers 30 during assembly. The backing layers 30 are preparedwith channels that are designed to distribute reactants to the outerface of catalyst-loaded gas diffusion layer 20. The backing layers 30also have frame regions to provide sealing surfaces. Fiber-reinforcedlaminate adhesive sheet 60 is patterned by cutting an open region withsurrounding frame to provide sealing around the perimeter while notobstructing the delivery of reactants to each gas diffusion electrode20. The open region is sized to accommodate the electrodes withoutadhesive blockage and the frame is sized to match the sealing regions bythe perimeter of the membrane electrolyte. The fiber reinforced adhesive60 could be prepared in sheet form which promotes ease of automatichandling (such as batch processing) as well as it allows for complexgeometric and intrinsic patterning. Fiber-reinforced laminate adhesivesheet 60 is preferably compliant by nature which increases durability ofthe fuel cell assembly, as well as it allows for optimization ofstiffness, toughness and other mechanical properties of the bond. Anexample of the fiber reinforced adhesive sheet is the Isola FR400series, e.g. FR402 pre-preg. However, other fiber adhesive agent couldbe used to achieve the objectives and advantages of the presentinvention to provide an adhesive as well as a fiber reinforced bond.

In an example of a fuel cell assembly, fiber-reinforced laminateadhesive sheet 60 is interposed between the electrolyte membrane 10 andeach of two flow distribution backings 30. The entire assembly issubjected to elevated temperate and pressure sufficient enough to allowthe fiber-reinforced adhesive to cure. Examples of elevated temperatureand pressure are for instance, but not limited to, 120 degrees Celciusand 900 kPa, respectively. An example of curing time is for instance,but not limited to, about 2 hours at 120 degrees Celcius and 900 kPa.The bonding step further allows the fibers within the adhesive sheet topenetrate into membrane surface 10, thereby greatly enhancing intimatecontact and mechanical interlocking (See FIG. 15). The full assemblyretains in bonded condition after the temperature and pressureapplications are removed.

FIG. 14 is a magnified side view of a section of an exemplary embodimentof fiber-reinforced laminate adhesive sheet 60, which preferablyincludes a network of fibers 70 and an adhesive material 80. In apreferred embodiment of the present invention the adhesive is retainedin the fiber matrix at room temperature, but liquid at elevatedtemperature as it would be applied during bonding.

FIG. 15 is a magnified cross-section side view of the fiber-reinforcedlaminate adhesive sheet, shown at the interface between an electrolytemembrane 10 and a flow distribution plate 30. The flowable adhesive 80forms a surface bond with flow distribution plate 30. However, ofparticular note for the present invention is the interface region 90, inwhich fibers 70 penetrate into the main body of the membrane electrolyte10. The penetrated fibers provides a mechanically enhanced bonding thatis superior to surface adhesion alone. The fiber-reinforce bond betweenmembrane electrolyte 10 and fiber-reinforced adhesive 60 accomplishessome deformation of a membrane electrolyte 10, which enhances the bondstrength even more.

The present invention has now been described in accordance with severalexemplary embodiments, which are intended to be illustrative in allaspects, rather than restrictive. Thus, the present invention is capableof many variations in detailed implementation, which may be derived fromthe description contained herein by a person of ordinary skill in theart. For instance, even though the present invention is described withregards to bonding between the membrane electrolyte and backing layers30, other components of the fuels cell assembly or even multiple fuelcells could be assembled using a similar approach of usingfiber-reinforced adhesive agents. Another variation is that thefiber-reinforced laminate adhesive sheet does not have to be supplied ina sheet form since it could also be supplied as a free-flow adhesive.The free-flow adhesive lends itself to alternative dispensing andpatterning strategies, such as robotic manipulation of a syringe tip.Yet another variation of the present invention is to simultaneously bondthe fiber-reinforced laminate adhesive sheet and the electrode/catalystbacking since both steps typically employ elevated temperature andpressure. For instance, a pre-assembled “hot-pressed” membrane-electrodeassembly is followed by a separate step of adhesive bonding. However, inprinciple some or all of these treatments may be performedsimultaneously. Still another variation is that the fiber-reinforcedlaminate adhesive agent could come as one or as multiple sheets that maybe stacked to achieve tunable thickness and bond characteristics. Stillanother variation of the present invention is to alter or combinedifferent integration process during fabrication of a fuel cell. Forexample, the fiber-reinforced adhesive agent may be patterned as anintegral of the flow backing structure, rather than introduced as aseparate sheet. It is also noted that patterning may include anymanufacturing process that distinguishes discarded regions from theportions to remain in the final application. This set of processesincludes but is not limited to stamping, blade cutting, laser cutting,photo-masking, and photo-developing. Still another variation of thepresent invention is that the method of assembling a fuel cell may bealtered to achieve preferred characteristics. For example, in apreferred embodiment both the cathode side and the anode side of themembrane are bonded simultaneously. However, for purposes that mayinclude special additives, seal inspection, humidity treatment,cleaning, etc. each side of the membrane may be bonded separately in anyorder. Still another variation of the present invention is thatselective electrical conductivity may be employed by eitherincorporating or eliminating conductive filler material, such as silverpowder or graphite fibers in the epoxy compound. All such variations areconsidered to be within the scope and spirit of the present invention.

Natural Bent Backing Layers

To keep the overall assembly height of a fuel cell 1 to a minimum, it isdesirable to make and assemble the individual layers as thin aspossible. At the same time, stiffness limitations of the used materialsdemand a certain minimal thickness where structural stiffness ismandated. Particularly and as it may be well appreciated by anyoneskilled in the art, the backing layers 30 that back the central layersof the fuel cell 1 has to provide sufficient stiffness either during apressure induced adhesive bonding operation and/or during operation ofthe fuel cell 1. To reduce the backing layers' 30 thickness, a naturalbent may be introduced to the backing layers 30 such as to counteractthe predetermined deformation of the backing layer 30. The predetermineddeformation may occur during the bonding operation and/or during fuelcell operation

As illustrated in FIG. 13, the backing layers 30 have a natural bentprior to their assembly. The natural bent is adjusted to forces Fapplied in the bonding areas 62. As a result, a contact pressure acrossthe area 63 is substantially equal between the assembled backing layers30, despite resilient deflection occurring in the consecutivelyassembled backing layers 30.

Proton Cross Conductivity Avoidance

According to FIG. 16, proton cross conductivity occurs between adjacentand electrically linked cell elements of a fuel cell 1. In FIG. 16 acathode diffusion layer 20 of cell A is electrically linked via lead 391to an anode diffusion layer 20 of cell B, not necessarily by penetratingthe interposed electrolyte layer, but eventually through an externalelectrical circuit. Due to the proximity of the two adjacent cell andthe proton conductivity of the membrane layer 10, protons propagatebetween the cathode diffusion layer 20 of cell A and the anode diffusionlayer 20 of cell B and a parasitic voltage potential arises between thecells A and B. The parasitic voltage potential has a degrading influenceon the operational cell voltages (OCV) of the affected cell elements.Proton conductivity depends on the structural configuration of themembrane between adjacent cell elements and on the gap distance betweenthe adjacent cell elements and the cell border area of each involvedcell element. The cell border area in context with the present inventionis the area defined by the border length times the height across bothanode and cathode diffusion layers 20 including the membrane thickness.FIG. 25 shows a graph for operational cell voltage (OCV) of cell A overvoltage potential of cell B experimentally determined for a cell gap of13mm (circle curve), for a cell gap of 4mm (solid triangle curve), andfor a broken membrane considered as ideal proton insulator (solid squarecurve). FIG. 26 shows various maximum power reduction for a solidmembrane in dependence of gap distance 5 min, 13 (see FIGS. 12, 17) overcell border area t*L₂.

Best proton insulation is provided by interrupting the path of protonpropagation. This is either provided by structural damaging of themembrane or by positioning an proton insulation structure betweenadjacent cell elements of said fuel cell across a material separation 12(see FIG. 12) of the electrolyte carrier membrane. As shown in FIGS. 12and 17, such proton insulation structure may be a fiber-reinforce resin61. A proton insulation structure may also by provided by a shapedlayer. Structural damaging may be accomplished with well-knownmechanical and/or chemical material removing techniques such as, forexample, stamping or etching.

Accordingly, the scope of the invention described in the specificationabove is set forth by the following claims and their legal equivalent:

1. A fuel cell having a number of in-plane arrayed cell elementsassembled in a substantially proton insulation fashion to each other,wherein said proton insulation is provided by an insulation structurepositioned between adjacent cell elements of said fuel cell across amaterial separation of an electrolyte carrier membrane, wherein saidinsulation structure is a shaped layer made of photo-sensitive material.2. The fuel cell of claim 1, wherein said insulation structure furthercomprises a predetermined selection of a spacing between adjacent andconductively linked cell elements.
 3. A fuel cell having a number ofin-plane arrayed cell elements assembled in a substantially protoninsulation fashion to each other, wherein said proton insulation isprovided by a insulation structure positioned between adjacent cellelements of said fuel cell across a material separation of anelectrolyte carrier membrane, wherein said insulation structure is anadhesive.
 4. The fuel cell of claim 3, wherein said adhesive is fiberreinforced.